The present invention relates to an antenna device achieving desirable transmission and reception in two or more frequency bands, and which is used in mobile communication apparatus such as cellular phones.
FIG. 14 shows one example of a conventional antenna device used in mobile communication apparatus such as cellular phones. In FIG. 14, 102 is a transmission and reception unit, and surrounding it is a main housing 101. On the top of the main housing 101 are a pair of antenna coils 103 and 104 which are disposed integrally on the inside of a common housing 110.
The two antenna coils 103 and 104 are disposed on the same axis. The antenna coil 103 placed on the top is connected to a transmission unit 105 of the transmission and reception unit 102 via a duplex filter 107. The antenna coil 104 placed at the bottom is connected to a reception unit 106 of the transmission and reception unit 102 via the duplex filter 107.
The electrical lengths of the two antenna coils 103 and 104 are designed such that the electrical lengths of the top antenna coil 103 and bottom antenna coil 104 are respectively a quarter of the transmission frequency and reception frequency of the transmission and reception unit 102.
The antenna coil 103 for transmission is disposed above the antenna coil 104 for reception. By positioning the antenna coil 103 further from metallic parts or other similar parts of the apparatus, better transmission can be obtained.
The two antenna coils 103 and 104 share a common feed line 108 to the duplex filter 107. The length of a transmission feed line portion 108a of the transmission antenna coil 103, which extends via the reception antenna coil 104 is open in the reception band, and is connectable with the duplex filter 107 in the transmission band. In FIG. 14, 109 is a second reception antenna.
Therefore, when the reception antenna coil 104 is used, the transmission antenna coil 103 does not place any load on the junction point, thus only the reception antenna coil 104 is driven. The antenna coils 103 and 104 are designed to match at 50 ohms so that impedance matching is not required.
With the above-mentioned conventional construction however, when trying to obtain the optimal impedance properties of the antenna, relative positions of the two antenna coils can not be changed and possible range for impedance adjustment is limited since the two antenna coils are both supplied with electricity and fixed.
Therefore, when the impedance for the antenna terminals of the duplex filter is changed due to a design change of a high frequency circuit inside the apparatus, the antenna device itself requires a design modification to deal with changed impedance. As such, rapid countermeasures to the design change of the apparatus are difficult to implement.
To mass-produce the device, the two antenna coils must be disposed, maintaining their relative positions precisely so that dispersion of the impedance of the antenna device does not occur. At the same time, the two antenna coils must be electrically connected to the high frequency circuit inside the apparatus. Thus, with this construction it is difficult to mass-produce the device productively.
The present invention aims at providing antenna devices and mobile communication apparatus using the devices, which allow an easy and wide-ranging adjustment of the impedance properties of the antenna as well as mass production of related products with good yield.
An antenna device of the present invention includes a) a spiral-shaped first antenna element of which one end is open and the other end is electrically connected with a high frequency circuit inside of a communication terminal; and b) a second antenna element having both ends being open, and which is insulated from and disposed on either outer or inner surface of the first antenna element. The impedance properties of an antenna can be adjusted by changing the disposing position of the second antenna element. Desired impedance properties can be gained just by changing the disposing position. A wide range adjustment of the impedance properties is also possible. Moreover, the construction of the antenna device is remarkably simple, thus its mass-production is easy.